Active charcoals are products resulting from the carbonization of various carbonaceous materials, wood, coal or polymers, which have been subjected to an activation treatment which confers on them a high porosity and a high adsorbent power. These adsorbent properties have been exploited for a long time for the treatment, separation and purification of gases and for the decoloration, purification and deodorization of liquids.
When they are brought into contact with liquids in non-continuous processes, they are generally used in the form of powders, with a mean particle size generally of less than 100 .mu.m, because the powders make it possible to achieve an active charcoal/liquid intimate contact which is very efficient. In percolation treatment processes, however the active charcoal is instead used in the form of rods or granules, in order to avoid any untimely movement of the adsorbent layer.
When the active charcoal is brought into contact with gases (adsorbent beds), excessively fine particle sizes are avoided, otherwise, the pressure drop would be excessively high and the risks of blockage of the equipment, would be increased.
It is possible to prepare the active charcoal by the action of a dehydrating and/or oxidizing agent (phosphoric acid, zinc chloride) on ligneous materials, (such as wood, coconut shells or coal), the operation being carried out without prior carbonization at a relatively low temperature (less than 600.degree. C.), the chemical substance subsequently being removed from the active charcoal by successive washing operations and, if necessary, grinding to the desired particle size. The active charcoals thus obtained (so-called chemical charcoals) exhibit a good porosity but their mechanical characteristics, which are often too weak (excessively friable grains), limit their use in the treatment of gaseous phases.
One method for the preperation of agglomerated active charcoal consists in mixing a hydrocarbonaceous material, such as coal, peat, peat coke, semicoke or wood charcoal, with coal pitch, coal tar, wood pitch or wood tar, in then agglomerating the mixture by compacting and, finally, in crushing it or extruding it through a die to the desired particle size. The agglomerated products are subsequently devolatiled at a temperature of 400-600.degree. C. before activation by gasification in an oxidizing atmosphere (steam, CO.sub.2 or H.sub.2 O/CO.sub.2 mixture). If the starting hydrocarbonaceous material is composed of fruit shells, for example coconut shells, it is not necessary to mix them with pitch or tar before the devolatilization and activation stages because grains in the desired particle size range, typically of the order of a mm, are obtained by simple crushing (the active charcoals prepared according to the latter method are so-called physical charcoals). The porosity of the active charcoal is created during the activation stage (with the consumption of carbon), which porosity goes together with the weakening of the skeleton of the graing, thus resulting in a significant loss of mechanical properties (decrease in the resistance to crushing, increase in the friability).
Provision has also been made to prepare agglomerated active charcoal by agglomerating active charcoal powder with peptized alumina or else with certain clays; once the active charcoal powder has been mixed with the alumina or the clay, the mixture is extruded and then subjected to baking at a temperature of between 450 and 600.degree. C. As it is known that, at such temperatures, active charcoal is capable of being consumed on contact with air or oxidizing fumes (used in direct heating furnaces), it is essential to maintain the above-described mixture under a neutral atmosphere, which increases the cost of this agglomeration method.
The active charcoal agglomeration process disclosed in FR 2,228,031 consists in mixing a finely divided carbonaceous material (with a particle size of less than 0.075 mm) with a thermosetting binder based on phenol/aldehyde resin and/or their condensation products, optionally in the presence of a crosslinking agent, in then moulding the mixture, in crosslinking the binder under hot conditions and, finally, in activating the thus agglomerated carbonaceous material.
Provision has been made, in FR 2,215,461, to agglomerate active charcoal powder with a thermosetting binder, which can be chosen from phenol-formaldehyde resins, furan resins or urea-formaldehyde resins, and then to shape the mixture, before carrying out a heat treatment, so as to cure the binder.
During the shaping of the agglomerates from powdered active charcoal and from a binder (thermosetting binder, pitch, tar, and the like) according to the prior art, it is found that a not insignificant part of the binder enters inside the pores, decreasing the activity of the active charcoal thus prepared.
In JP09-156,914 is disclosed a process for agglomerating an active charcoal powder with a binder, whereas prior to the agglomeration step, the active charcoal is brought into contact with a liquid pore-protecting agent selected from water, higher alcohols, fatty acids, oil/fats and plasticizers. The agglomerates that are obtained have a poor resistance to crushing.